1. Field
The embodiments discussed herein are directed to a plasma display panel (PDP), and structure of a sealing material that is used for sealing a front substrate and a rear substrate.
2. Description of the Related Art
A plasma display panel includes a front substrate and a rear substrate, which are both larger than a screen. The front substrate and the rear substrate are opposed to each other and sealed with a sealing material that is arranged at the outer portion of the screen and has a frame-like shape so that a closed discharge gas space is defined by them. The front substrate and the rear substrate are glass substrates, while the sealing material is a burned material of low melting point glass.
Among plasma display panels having such a structure, a surface discharge type plasma display panel for use as a color display includes partitions that prevent discharge interference between neighboring cells. The partitions divide the discharge gas space into plural spaces and define a thickness of a portion of the discharge gas space corresponding to the screen. Arrangement patterns of the partitions include a stripe pattern and a mesh pattern. According to the former arrangement pattern, the discharge gas space is divided into plural columns of a matrix display. According to the latter pattern, the discharge gas space is divided into cells of plural columns and plural rows.
In a plasma display panel with partitions, there can be generated a slight curvature of either the front substrate or the rear substrate or the both of them after they are sealed. For example, in a burning process for melting and hardening the sealing material or in a vacuuming process for cleaning the inside prior to filling discharge gas, the pair of glass substrates can be curved by actions of temperature rise of the glass substrates and pressure reduction inside so that the sealing material is compressed. As a result, a thickness of the plasma display panel becomes smaller than a design value at the sealing portion between the front substrate and the rear substrate, while it becomes larger than the design value at the peripheral portion of the screen inside the sealing portion. There can be generated a gap of approximately 10 microns between the partition and the surface of the substrate that are to contact each other inside the portion where the thickness of the plasma display panel becomes larger than the design value. A region with such malcontact may appear in a frame shape along the edge of the screen with a width of approximately a few centimeters. Hereinafter, the decrease of the thickness of the plasma display panel at the sealing portion is referred to as “subsidence”.
The malcontact between the front substrate and the rear substrate inside the sealing portion may cause an abnormal noise during a display operation. When a high frequency drive voltage is applied for a display, periodical electrostatic attraction may vibrate the glass substrates locally, so that a low level of noise at an audible frequency is generated. This noise may deteriorate quality of display operation.
Regarding a method of preventing the curvature of the front substrate and the rear substrate, Japanese unexamined patent publication No. 2001-236896 discloses a sealing material that includes glass beads as spacers. The spacers have substantially the same size of diameter as a height of the partition, so that the gap between the front substrate and the rear substrate at the sealing portion can be maintained at a desired value.
It is necessary that the sealing material includes a sufficient quantity of spacers between the front substrate and the rear substrate along the entire perimeter of the sealing portion in order to make the thickness of the plasma display panel uniform. If a quantity of the spacers is insufficient, the spacers may be broken by an excessive pressure per spacer.
However, if a quantity of glass beads contained as the spacers in the sealing material is increased, viscosity of glass paste that is the sealing material before being burned increases. As a result, productivity in applying the glass paste may be lowered, and height as well as width of a layer of the applied paste tends to be nonuniform. In particular, if glass beads having a broad distribution of granularity are used, viscosity of the glass paste may increase largely.
It may be desirable to use glass beads having uniform grain size without smaller grains that do not work as spacers in order to prevent the increase in viscosity. However, a classification work for obtaining glass beads of a sharp distribution of granularity causes increase of cost of the glass beads. It may be difficult to remove smaller particles compared with removal of larger particles than a desired size.